Protein Synthesis
The code held in DNA gives the cell instructions for making protein. The code is in the form of base triplets called codons. There are four possible bases in DNA, given the abbreviations A, T, C & G. Each triplet of bases translates to a particular amino acid in the sequence. For example the base triplet CAT codes for the amino acid Valine while TCA is Serine. There are also special codes which tell the cell where the sequence for the protein starts and stops.
When the cell needs to make a protein a copy of the gene is needed as the DNA always remains inside the nucleus of the cell. The DNA uncoils partially and the gene acts as a template for individual nucleotides to match up with the bases just like in DNA replication. There is one important difference. The nucleotides are RNA (ribonucleic acid) and not DNA. They have four bases just like DNA with the exception that base Thymine is replaced by a similar base Uracil (U). As the RNA bases match up with the DNA bases of the gene, an enzyme joins up the separate bases to form a single strand of RNA known as messenger RNA. This mRNA leaves the nucleus and heads for the rough endoplasmic reticulum (RER). This process is known as transcription.
The ribosomes on the RER are made from protein and ribosomal RNA and their function is to co-ordinate protein synthesis. In the cytoplasm around the RER are transfer RNA molecules. There are different tRNA molecules, each with a codon made up from a triplet of bases. The codons are complementary to those on the mRNA. Each tRNA molecule has an amino acid attached to it. The codon on the tRNA molecule determines which amino acid is attached. The tRNA molecules match up with their complementary base codon on the mRNA molecule. The sequence of bases in the mRNA molecule determines the sequence of tRNA molecules which match up. The sequence of tRNA molecules determines the sequence of amino acids. An enzyme links the separate amino acids to form a continuous chain and thus a protein is made. This process is known as translation.
